1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium. The image forming apparatus includes, for example, an electrophotographic image forming apparatus employing an electrophotographic method. Specifically, the image forming apparatus includes a laser beam printer, a copying machine, and a facsimile machine.
2. Description of the Related Art
An electrophotographic image forming apparatus uniformly charges a surface of a photosensitive member serving as an image bearing member with a predetermined polarity, and forms an electrostatic latent image based on image data. The electrostatic latent image is developed with toner (developer) to form a toner image (a developer image). This toner image is transferred to a recording medium such as a sheet, and is fixed to the recording medium by heat and pressure in a fixing device.
As for a charging unit, a roller charging method using a conductive roller for a charging member is widely used.
A charging roller serving as the conductive roller has proper elasticity to be in contact with a photosensitive drum at a certain level. When a certain voltage or higher is applied while the charging roller is being in contact with the photosensitive drum, a surface potential of the photosensitive drum begins to rise and then linearly increases according to the applied voltage.
When this threshold voltage is defined as a charge start voltage Vth, a direct current (DC) voltage of V0+Vth needs to be applied to a charging roller to acquire a photosensitive member surface voltage (hereinafter referred to as a primary charging potential Vd0) needed for an electrophotographic image forming process. Such a charging method is referred to as a direct current charging method.
In the direct current charging method, for example, Japanese Patent Application Laid-Open No. 8-171260 discusses a technique as one method for enhancing uniformity of a surface potential of a photosensitive member. Such a technique is described with reference to FIG. 8.
In Japanese Patent Application Laid-Open No. 8-171260, the photosensitive member is once overcharged to a primary charging potential Vd0 that exceeds a potential needed for image formation by a primary charging device. After the primary charging and before development, an exposure device emits a weak light at a power E1, thereby attenuating (decreasing) the surface potential of the photosensitive member.
Accordingly, potential of the photosensitive member becomes Vd1 which is a target value during image formation. This potential Vd1 is provided in an area where adhesion of toner is not allowed (i.e., a non-image portion) within an image forming area of the photosensitive member. The potential Vd1 is hereinafter referred to as non-image portion potential.
In an area where adhesion of toner is allowed (i.e., an image portion) on the photosensitive member, image portion potential V1 is provided by light emitted at a power E2 by the exposure device.
According to Japanese Patent Application Laid-Open No. 8-171260, this method can prevent generation of a defective image as a conventional problem of the direct current charging method due to overcharge of a photosensitive member.
Meanwhile, potential of the photosensitive member also depends on the sensitivity of the photosensitive member. There are cases where the sensitivity of the photosensitive member changes depending on an amount of light to which a latent image is exposed. For example, when images are formed on different sizes of recording media, a width (a length in a longitudinal direction) on a photosensitive member to be exposed to light is changed so that an electrostatic latent image is formed. That is, a total amount of light exposure changes in a photosensitive member longitudinal direction. Accordingly, there are cases where a change in the sensitivity of the photosensitive member changes potential of the photosensitive member in the longitudinal direction thereof. That is, density unevenness occurs.
Japanese Patent Application Laid-Open No. 4-22977 discusses prevention of the density unevenness by exposing an outer side, which is provided outside of a width in which an electrostatic latent image is formed on a photosensitive member, to light during image formation when a recording medium is small.
An exposable width on the photosensitive member exposed to light by an exposure device can satisfy a width in which a maximum image is formed. In addition, a developing width on the photosensitive member can be wider than the exposable width. The developing width is a width in which a developing device can perform development. Moreover, in a case where an uncharged portion is generated in an area of the photosensitive member corresponding to the developing width, the uncharged portion of the photosensitive member is developed. Thus, a charging width on the photosensitive member is wider than the developing width. The charging width is a width in which a charging device charges a surface of the photosensitive member. Therefore, the following relationship is provided.Maximum image forming width<exposure width<developing width<charging width
However, in a potential control method where an exposure device emits a weak light subsequent to a primary charging to acquire non-image portion potential Vd1, an exposure width and a width of the non-image portion potential Vd1 become the same. Accordingly, the following relationship is satisfied.Maximum image forming width<exposure width=non-image portion potential Vd1 width<developing width<charging width
The developing device includes a developer bearing member for carrying toner serving as developer, and the developing device employing a developing roller method is widely used. Potential used to develop toner on a photosensitive member (hereinafter called a developing bias Vdc) is being applied to a developing roller.
Herein, a relationship among the primary charging potential (a potential of a photosensitive member charged by a charging unit) Vd0, the non-image portion potential (a potential of the photosensitive member exposed to a weak light) Vd1, and the developing bias (a potential of a developing roller) Vdc is as follows:|Vdc|<|Vd1|<|Vd0|An electric potential relationship between the developing bias Vdc and the non-image portion potential Vd1 or the primary charging potential Vd0 has the following restrictions.
First, if a potential difference between Vdc and Vd1 is too large, a toner charged to a polarity opposite to a normal charge polarity is moved to an area of the photosensitive member having the potential of Vd1. Such a toner movement is so-called reversal fog. As a result, a defective image (i.e., a fogged image) caused by adhesion of the toner to a non-image portion is generated. Moreover, if a potential difference is too small, a toner charged to a normal charge polarity is moved to an area of the photosensitive member having the potential of Vd1, causing generation of a fogged image as similar to the case of the big potential difference. Thus, an electric potential relationship between the non-image portion potential Vd1 and the developing bias Vdc in an image region needs to be set appropriately.
However, when there is a conventional length relationship stated below in a photosensitive member longitudinal direction,Non-image portion potential Vd1 width<developing width<charging widththe photosensitive member remains charged by the primary charging potential Vd0 on an outer side of the non-image portion potential Vd1 width in an area of the developing width.
Consequently, the potential difference between the developing bias Vdc and the primary charging potential Vd0 increases, thereby causing the reversal fog which is adhesion of the toner charged to a polarity opposite to a normal charge polarity to an area having the potential Vd0 of the photosensitive member. Accordingly, the fog occurrence on an end in a photosensitive member longitudinal direction can cause the possibility that a stain is generated on an edge of a sheet serving as a recording medium.
Next, a description is given of another issue regarding an outer side of the non-image portion potential Vd1 width in an area of a charging width where the following conventional length relationship is provided in a photosensitive member longitudinal direction.Non-image portion potential Vd1 width<developing width<charging width
The potential outside the non-image portion potential Vd1 width in an area of a charging width on the photosensitive member is the primary charging potential Vd0. The non-image portion potential Vd1 is provided by attenuating (decreasing) potential of the photosensitive member from a value of the primary charging potential Vd0 by exposure of a weak light by an exposure device (weak exposure device). This resolves the issue of the direct current charging method.
Therefore, the potential of the photosensitive member is attenuated by the weak light emitted by the exposure device. The greater the attenuation amount, the more the effect. When an absolute value of the primary charging potential Vd0 is greater, the weak light of the exposure device can provide a greater amount of attenuation potential of the photosensitive member.
However, when an absolute value of the primary charging potential Vd0 is greater, the photosensitive layer has the possibility of undergoing dielectric breakdown. The photosensitive member includes a photosensitive layer formed on a surface of a cylinder made of metal such as aluminum. The photosensitive layer serves as the photosensitive member. This photosensitive layer is an insulator, and electric charge charged by primary charging is retained on a surface of the photosensitive layer.
When electric charge on the surface of the photosensitive member is greater, there is a possibility that withstand voltage of the photosensitive layer is exceeded, causing dielectric breakdown of the photosensitive layer. Moreover, when electric charge is retained on the surface of the photosensitive member for a long period, there is a possibility that dielectric breakdown of the photosensitive layer occurs. In case of dielectric breakdown of the photosensitive layer, an electric current at the time of primary charging by a charging member flows into the area having undergone the dielectric breakdown. As a result, a voltage necessary to charge the photosensitive member by the charging member cannot be maintained, thereby causing generation of an image with faulty charging.